This research is an attempt to reconcile the predicted travel time savings computed in the SPOT process, using various analysis tools as part of the statewide project prioritization scheme at NCDOT. The tools considered include the NC Statewide transportation model (NCSTM), the microsimulation tool Trans Modeler (CMT), and a sketch planning tool named CALC.
Data used for this research included a random sample of 43, P5 projects that focused on specific improvement types, or SIT 1, 5, 8,10,11,16 and 17, as relayed by the project steering committee. The bulk of the work consisted of improving the CALC approach to make it more defensible, and bring it closer in methodology to the statewide model approach. This applied to all except for SIT8 and 10 projects conducted via simulation.
For the previous P5.0 projects as well as the in-process P6.0 projects, meta- analysis of the project inputs found that individual SITs displayed unique trends in existing vs. project number of lanes, speed limit, length, and volume to capacity (V/C) ratio. Specifically, projects with widening or other capacity improvements showed clear clustering of V/C values while projects adding new roadways had much more varied V/Cs.
Regarding SIT8 and 10 use in Trans Modeler, the team made several recommendations to streamline the calculation process, and make it more transparent in terms of inputs and outputs, including the use of adjustment factors that require some justification. In addition, the team has provided project-specific comments regarding some of the model outputs and possible changes in the methods currently being used.
Proposed TransModeler Process for 10-Year TTS Estimation
Three major modifications are proposed in the CALC procedure as a result of this research. First, the team proposes the use of facility-based congestion factors, using the latest HCM6 methods for freeways, multilane highways, two lane highways and arterials. This approach is also consistent with the way CALC develops HCM-based capacities using NCLOS. Second, regarding SIT-5, the team proposes the use of a simplified assignment model to predict the amount of AADT that will be diverted to the new project location, and that which will remain on the existing facility, and calculate travel time savings on each facility separately. And finally, the team proposes to use a different method for calculating the peak to daily (PTD) factor based on NCSTM data. Finally, the project team has generated an Excel based tool that can carry out both the current/ existing and the new proposed CALC methods for either individual or a group of projects, along with a user guide for its use.
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